1. Field of the Invention
This invention relates, in one aspect, to a brightness measuring apparatus and more particularly to a brightness measuring apparatus adapted to measure brightness in a plurality of areas in an object field and find the brightness of a main object. In another aspect, this invention relates to a system for flash photography apparatus.
2. Description of the Related Art
Several cameras have heretofore been proposed in which a photographing field or scene is divided into a plurality of areas for achieving the proper exposure of the main object to be photographed.
For example, Japanese Patent Laying-Open No. 58229/1987 discloses a camera which is so designed that a scene to be photographed is divided into a central area and a peripheral area with said central area being further subdivided into two areas, top and bottom, and which is equipped with a brightness measuring device adapted to measure brightness in these areas independently. In photographing with this camera under ambient light conditions, the rear light condition is detected and the exposure controlling value calculated on the basis of either the smaller (darker) (hereinafter referred to as brightness data BV.sub.1) of the two values obtained in the respective central areas or the value (hereinafter referred to as brightness data BV.sub.3) determined from said brightness data BV.sub.1 and the brightness value obtained in the peripheral area (hereinafter referred to as brightness data BV.sub.2). To be specific, in this camera, when the ambient light condition is dark with the value of brightness data BV.sub.1 being lower than a predetermined level, the detection of rear light and the calculation of the exposure controlling value are performed according to said brightness data BV.sub.3 (average brightness value). Otherwise, the detection of rear light and the calculation of the exposure controlling value are performed according to brightness data BV.sub.1.
However, when, as in the prior art camera described above, the central zone of the brightness measuring area is divided into a plurality of areas and brightness measurements are independently performed in these areas, the outputs of photosensor elements in the central areas may be increased beyond the true values, under rear light conditions, by background brightness and cross talk among the sensor elements. Furthermore, the absolute value of this error varies with the positional relation between the area of brightness measurement and the main object (cf. FIG. 12).
Therefore, when the central area is thus divided into more than one areas and the average of the brightness values in these central areas is employed, the rear light detection cannot be accurate. Moreover, if this average value of brightness in the central area is regarded as the brightness of the main object, the value will not represent the true brightness of the main object.
On the other hand, if the brightness of the main object is determined from one of the measured brightness values in the central areas, an error will be inevitable owing to the reflectance, which may be varying, of the main object. Particularly under front light conditions, the background brightness will not be a major influential factor but the influence of reflectance will be remarkable.
Furthermore, Japanese Patent Laying-Open No. 58229/1987 discloses a flash photographing system which performs exposure control so as to overexpose the background under rear light conditions and actuates a flashlight emission to insure the proper exposure of the main object and, at the same time, allow an impression of rear light photography to be reproduced in the picture. In flash photographing with the above photographing system under front light conditions (where the difference in brightness between the main object and the background is small or the main object is brighter than the background), the main object is overexposed at times because, in this system, exposure control is made to insure the proper exposure of the background. Particularly in a situation where the main object is brighter than the background, even ambient light alone causes an overexposure of the main object so that as flash light is superimposed on ambient light, the main object is markedly overexposed.
Still further, there have been proposed cameras which judging whether rear light photography or front light photography is to be carried out. These cameras have means for measuring brightness of the center area, means for measuring brightness of the peripheral area, and means for judging the scene to be photographed is rear light condition when the brightness of the peripheral area is high by a predetermined value (rear detection level) than that of the central area. But, in such conventional cameras, since the rear light detection level is fixed, it is not always to accurately detect the rear light condition. For example, in the conventional cameras, it is not possible to accurately detect the rear light condition when the proportion of the object in the light measuring area is pretty small or pretty big.